Correct order of ossl_condvar_signal in quic_multistream_test

Correct order of ossl_condvar_signal in quic_multistream_test

quic_multistream test was issuing a signal on a condvar after dropping
the corresponding mutex, not before, leading to potential race
conditions in the reading of the associated data

Fixes #22588

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22616)